CENTRONELLA 241 



of the shell, in this respect simulating the genera Pugn-ax and Pugnoides. 

 In fact, were it not for the punctate shell structure the species would be 

 placed in the genus Pugnoides without hesitation. It is possible, of 

 course, that more than one generic group of rhynchonelloid shells de- 

 veloped the punctate structure, and that all such shells should not be re- 

 ferred to Rhynchopora, but until the internal characters of the genotype 

 have been determined, it seems unwise to erect any additional genera. 

 Horizon. Okaw limestone of the Chester group. 



Family CENTEONELLID^ 

 Genus CENTRONELLA Billings 



Description. Shell small, rostrate, plano-convex or concavo-convex, with 

 smooth valves. Pedicle valve with a median, longitudinal ridge from 

 which the surface slopes more or less abruptly to the lateral margins, 

 the beak acute and incurved, perforated at its apex by a subcircular fora- 

 men, the delthyrium closed by a pseudodeltidium. Brachial valve con- 

 cave transversely by reason of the presence of a median sinus which is 

 not apparent on the interior, the beak small, not incurved. Internally the 

 dental sockets are broad, bounded interiorly by the high walls of the 

 hinge-plate. This plate is divided medially by a deep furrow extending 

 to the apex, and, therefore, consists of two processes which are elevated, 

 thickened and rest on the bottom of the valve. From the anterior face 

 of these arise the crura which converge for a short distance, and expand 

 to form two broad, acute jugal processes. From here the lateral branches 

 of the brachidium curve outward, gradually turning from a vertical to a 

 horizontal position, broaden rapidly and unite to form an anterior trian- 

 gular plate which bears a median ridge where the two lateral branches 

 are conjoined. 



Remarks. The species which are here referred to the genus Centron- 

 ella are so placed because of external form alone. The description of the 

 brachidium given above is from the work of Hall and Clarke, and it is by 

 no means certain that the characters of these Mississippian species 

 conform to the definition of the genotype. 



CENTRONELLA LOUISIANENSIS n. sp. 



Plate XXX, Figs. 26-29 



Description. Shell minute, subovate in outline, longer than wide, the 

 greatest width anterior to the mid-length, the anterior and antero-lateral 

 margins describing nearly a semicircle or slightly truncated in front, the 

 postero-lateral margins nearly straight or gently convex, meeting at the 

 beak in an acute angle. The dimensions of a perfect specimen are: 

 length of pedicle valve 2.9 mm., length of brachial valve 2.5 mm., width 2.7 

 mm., thickness 1 mm. 



